In digital systems, pulses which must be delayed in accordance with a clock signal or must occur in a specific phase position are very often required. Such pulses are needed, for instance, in digital data transmission in order to decode a self-clocking signal. The recording of a binary coded data signal on one longitudinal track of a video tape, where this signal is representative for every location on the tape, is known from the Journal of the SMPTE, June, 1973, Vol. 82, pages 482-491. This data signal is recorded in the form of a continuous pulse train, as a sequence of 80-bit words, and it contains a time signal, in the form of an indication of hours, minutes, seconds, and frame number; application-oriented information; and a synchronizing signal. The time signal and the application-oriented information take up 64 bits, and the synchronizing word takes up the remaining 16 bits. The bi-phase mark code, a self-clocking code, has come to be the worldwide standard for encoding this data signal. In this code, a logical "0" is represented by a phase alternation at the end of a bit cell and a logical "1" is represented by an additional phase alternation in the middle of the bit cell.
In order to decode a signal encoded in the bi-phase mark code, the basic clock, that is, each phase alternation at the end of one bit cell, must be recognized. From the basic clock, a gating pulse must be obtained, which isolates the additional phase alternation of a logical "1" and thus converts the signal into a bit sequence of "0" and "1". In this process it is useful to generate a basic clock signal having a duty cycle of 0.75:1, in order to isolate the additional phase or edge alternation in the middle of the bit cell. Circuits for decoding such a signal are known per se. Since this SMPTE signal is recorded in magnetic tape recorders as a time code mark or marking on the tape, however, it undergoes changes in frequency of several decades between slow and fast rewinding, which makes the decoding more difficult.
A method for generating the basic clock on an analog basis is already known, in which the amplitudes of sawtooth wave pulses, which have a duration of one bit cell, are measured. A comparator compares the sawtooth wave amplitude at 75% of the period and thus furnishes the corresponding code clock signal. The disadvantage of the known method is that with increasing clock frequency, the sawtooth wave amplitudes become smaller, and evaluation thus becomes more difficult.
It is also known to obtain the basic clock by means of digital circuitry, by measuring the duration of the previous bit cell with a counter, and during the counting by the counter in the next bit cell to compare the counter state with 75% of the maximal counter state. The disadvantage here is that the circuitry for realizing the counter is complicated and thus extensive in scope and hence costly.